The mono-crystal furnace mentioned in the invention is a furnace for growing monocrystalline silicon.
The multi-crystal furnace mentioned in the invention is a furnace for growing polycrystalline silicon.
The raw material silicon brick mentioned in the invention is the bulk unmelted silicon placed into a mono-crystal furnace or multi-crystal furnace, and is the raw material for the production of monocrystalline silicon or polycrystalline silicon.
A description of the related prior art follows:
First, it strongly emphasizes that there is no existing literature, including U.S. Pat. No. 7,175,685, reporting that isostatic pressing technology could reduce the negative influence of oxide film on the quality of silicon crystal growth in a mono-crystal furnace or multi-crystal furnace. There is also no existing literature reporting that raw material silicon brick with good filling property produced from silicon powder by cold isostatic pressing or hot isostatic pressing is good for reducing oxide film on the surface and therefore good for reducing the negative influence on the quality of silicon crystal growth in mono-crystal furnace or multi-crystal furnace.
Oxides on the surface of raw material silicon brick will cause a series of problems for subsequent processes. For instance, silicon feedstock melts with difficulty; volatile matter reacts with equipment components; and the quality of crystal and final products, like chips, solar cells, etc. is reduced. So the existing technology needs to use chemical solution to clean off the oxides on the surface of raw material silicon brick.
The use of high-purity silicon brick as raw material in the existing technology usually involves the processes of high temperature melting and cooling, or the process of vapor deposition. So its relative density is 100%, and its real density is 2.33 g/cm3. Even though high-purity silicon brick is high-priced, its source is widely available and easy acquired. Silicon brick having a high density and good mechanical properties can absolutely tolerate the process of cleaning off oxides on its surface by chemical solution. After testing several raw material silicon bricks having a relative density of 100% and a real density of 2.33 g/cm3 from different sources, the compression resistance parameter is always within a range of about 60 Mpa to 206 Mpa with an average of 150 MPa. The related detailed description is explained in Table I.
However, raw material silicon brick produced from silicon powder by isostatic pressing is without the processes of high temperature melting and cooling, and its relative density is unable to reach 100%, so its impact resistance to tolerate chemical solution cleaning its surface is weaker than the traditional raw material silicon brick with real density of 2.33 g/cm3.
Silicon powder, such as the powder related to U.S. Pat. No. 7,175,685, easily contacts the air and generates oxide film, because the specific surface area of silicon powder is very large. If silicon powder, which can easily flow with gases and damage devices seriously, is charged into a mono-crystal furnace or multi-crystal furnace for growth, the quality of the produced monocrystalline silicon or polycrystalline silicon is poor making it very hard to meet market required quality.
Second, the technologies of charging silicon raw material into a silicon crystal growth furnace, have long been quite perfect. The silicon raw material is charged into a mono-crystal furnace to finish monocrystalline silicon growth or into a multi-crystal furnace to finish polycrystalline silicon growth. Monocrystalline silicon or polycrystalline silicon growth normally includes the processes of melting by heating, slow crystal growth and cooling.
The produced monocrystalline silicon or polycrystalline silicon can be used for solar cells after a cutting process. There is a quality standard abided by the international solar cell industry, so the monocrystalline silicon or polycrystalline silicon before cutting process also abides by the common quality standard. Technologies of monocrystalline silicon or polycrystalline silicon growth are largely identical but have some minor differences. Consequently, whether a large amount of cheap silicon raw material can be used with a guaranteed quality is a question of great concern to manufacturers of monocrystalline silicon or polycrystalline silicon.
Third, the prior art first introduces the application situation of silicon powder or silicon brick as raw material in mono-crystal furnace or multi-crystal furnace.
Application situation of silicon powder: on the one hand, silicon powder can easily flow with gases, which impacts the normal working of devices and the health of production staff. So its application has been severely limited. On the other hand, compared to silicon brick, the specific surface area of silicon powder is large, so it easily to contacts the air and oxidizes during the process of packing, transportation and storage. Furthermore, the oxidized silicon powder has a negative effect on the final product, namely, solar cells. This negative influence can be tolerated in the case of a small amount usage of silicon powder. Therefore, we can use a small amount of silicon powder in a mono-crystal furnace or multi-crystal furnace to reduce the costs. However, it is almost impossible to reduce the production cost greatly by using a large amount of silicon powder in mono-crystal furnace or multi-crystal furnace. The detailed description is explained in subsequent paragraphs.
Application situation of silicon brick: bulk silicon brick as the raw material is the usual form that is charged into a mono-crystal furnace or multi-crystal furnace.
Silicon brick utilized as raw material in a mono-crystal furnace or multi-crystal furnace of the solar or semiconductor field should meet two basic requirements. First, its purity should be within a range of 99.99% to 99.9999999%, preferably of 99.999% to 99.9999999%, and the best is within a range of 99.9999% to 99.9999999%. Second the silicon brick must have certain degree of compression resistance. In this case, silicon brick dust produced by moving the silicon brick should be minimized as it can easily damage devices or the health of operation staff.
If the purity of the silicon is too high, then it is uneconomical to be utilized in the solar field. For instance, the cost of silicon having a purity of 99.9999999999% utilized in the semiconductor field is extremely high.
If the purity is too low, the quality of silicon ingot produced from a mono-crystal furnace or multi-crystal furnace is too poor to meet the industry standard. A purity grade of silicon within a range of 99.99% to 99.999% is low. However, if mixed with other high-purity silicon feedstock, it can meet the quality requirement of silicon ingots. As the main raw material of the solar field, silicon feedstock having a purity within a range of 99.999% to 99.9999999% or 99.9999% to 99.9999999% is optimum.
It strongly emphasizes that silicon brick having a certain level of compression resistance and purity of 99.999% to 99.9999999% or 99.9999% to 99.9999999% is mainly produced by vapor deposition method. Silicon brick is especially produced from silicon compounds (like trichlorosilane, silane, etc) by chemical vapor deposition at high temperature. Its density is about 2.33 g/cm3. The detailed information is present in Table II.
Because high-purity waste silicon brick recycled from the semiconductor or solar industry has the process of casting, its density is also about 2.33 g/cm3 with superior compression resistance, and its purity is within a range of 99.9999% to 99.9999999999%.
Fourth, there is currently no appropriate method in the existing technology to apply silicon powder to mono-crystal furnace or multi-crystal furnace in large quantities.
The reasons are as follows:
Silicon (mono-crystal and multi-crystal) material with crystal structure has been widely used in semiconductor, solar, and integrated circuit industries. The silicon feedstock is usually produced by the processes of high-purity silicon feedstock melting and crystal growth. The utilized high-purity silicon feedstock is usually produced from silicon compounds (like trichlorosilane, silane, etc) by chemical vapor deposition at high temperature.
High-purity silicon feedstock produced by silicon feedstock manufacturers has two main forms, bulk/rod and rough graininess. Bulk/rod silicon feedstock is normally produced by breaking up high-purity big silicon rod or silicon ingot. Rough graininess silicon feedstock is produced by fluidized bed process of chemical vapor deposition. The diameter of these grains is normally within a range of several hundred micrometers to millimeter-sized. The two forms of silicon feedstock are convenient for transportation, and its tap density in crucible used for crystal growth is high whose filling factor is usually greater than 50%. Therefore, the two forms of silicon feedstock are widely utilized in industrial production.
Rough graininess silicon usually comes from fluidized bed process. It will also produce extremely fine silicon powder having a particle size within a range of a submicrometer to several hundred micrometers, which is the byproduct of rough graininess silicon. In addition, there is another kind of silicon powder having a particle size of micrometer and submicrometer grade produced by silane gas pyrolysis at a high temperature. Fine silicon powder usually relates to the process of cyclone dust collection or filter powder deposition, and the particle size range of the two kinds of silicon powder are usually not the same.
However, extremely fine silicon powder, currently the byproduct produced by fluidized bed process or other processes, is difficult utilized in the field of crystal growth or other fields. The main reasons are as follows: first, fine powder flows easily with gases. During crystal growth or other processes, the device is first vacuumized, then the appropriate protective gases are charged into the device. Because the particle size of the powder is small and light, it flows easily with the gases in every direction during the process of vacuumizing and charging protective gas. These powders damage the device component, as well as influence on the stability of the process and product, or can even lead to serious accident. Also, when operation staff breathes in flowing powder during the charging process, they are exposed to some occupational diseases like silicosis, etc. Second, tap density is small. The particle size of the fine powder is small so it has a low loose density. More room is needed during transportation and storage. More importantly, it is impossible to charge any more feedstock during crystal growth. Powder feedstock cannot be re-charged, since the powder flows easily with gases. Fine silicon powder for example, has a loose density within a range of 0.25 g/cm3 to 1 g/cm3, which is much lower than the pure silicon brick having a density of 2.33 g/cm3. The result is a very small charging capacity. A square crucible with internal size 69 cm×69 cm×42 cm can charge 240 kilograms to 300 kilograms silicon feedstock, but no more than 150 kilograms of silicon powder. Third, silicon powder easily oxidizes or has other reactions in air because the particle size of fine powder is small, but has a large specific surface area. Silicon powder easily oxidizes in air, and one layer of oxide film forms on the surface of the silicon powder. Furthermore, fine silicon powder easily gets damp in the air, and the adsorbed moisture will encourage further oxidation. These oxides will cause problems to subsequent processes. For instance: silicon feedstock melts with difficulty; volatile matter reacts with the equipment; and the quality of crystal and final products like chips and solar cells are reduced. Consequently, the utilization of extremely fine silicon powder in crystal growth is restricted.
For the above reasons, even though the purity of fine silicon powder produced by fluidized bed process is high, it has not been widely used. Currently, there is no effective way to utilize fine silicon powder.
In order to develop the application of silicon powder, the invention has done the following experiment. The steps are as follows. First, high-purity silicon powder having the period of transportation and storage for 4 months is charged into a mono-crystal furnace or multi-crystal furnace. The process of finishing crystal growth takes 30 hours to 60 hours by tolerating that flowing dust damages the device seriously. Then the silicon crystal is sliced, its properties are tested. The results show that the crystal cannot totally meet the quality requirements of solar cells. The silicon crystal must be charged into mono-crystal furnace or multi-crystal furnace again to finish the process of crystal growth for another 30 hours to 60 hours. Then the silicon crystal is tested once again. After the second processing in the furnace the quality requirements of solar cells are met. However, the cost processing the silicon crystal growth is unbearable for the manufacturers.
In recent years, the development of the semiconductor and photovoltaic industry, has worsened the serious shortage of silicon feedstock. Silicon feedstock has become a determinant of the development of these industries. Thus, a viable and feasible method to utilize these high-purity and extremely fine silicon powder effectively is highly sought after. This method is important to the reduction of production cost, competition enhancement, and promotion of the industry development, especially popularization of photovoltaic product in large scale.
Moreover, since it takes a long time to accomplish crystal growth process in a mono-crystal furnace or multi-crystal furnace, it is meaningful for production efficiency improvement and energy conservation to try to increase crucible charging capacity.
American U.S. Pat. No. 7,175,685 is a technology that is most similar with the invention.
In U.S. Pat. No. 7,175,685, it has found the importance of taking full utilization of silicon powder. U.S. Pat. No. 7,175,685 puts forward a technical solution to increase crucible charging capacity by increasing the density of silicon powder by simple dry pressing method. However, we found that the technical solution of U.S. Pat. No. 7,175,685 ignores the disadvantage that silicon bricks made by simple dry pressing method is easily cracked, and the problem of silicon powders falling off the local surface of silicon brick easily.
The abstract of U.S. Pat. No. 7,175,685. “A bulk silicon material for making silicon ingots, consisting of silicon pellets, and a method for making the pellets from an agglomerate-free source of high purity silicon powder by feeding a controlled amount of silicon powder that is free of intentional additives and binders into a pellet die, and dry compacting the powder at ambient temperature with pressure to produce a pellet that has a density of about 50-75% of the theoretical density of elemental silicon, a weight within a range of about 1.0 gram to about 3.0 grams and preferably of about 2.3 grams, a diameter in the range of 10 mm to 20 mm and preferably of about 14 mm, and a height in the range of 5 mm to 15 mm and preferably of about 10 mm.”
In claim 23 of U.S. Pat. No. 7,175,685, “The method for making a silicon pellet according to claim 13 wherein said step of dry compressing comprises applying a force greater than or equal to about approximately 10,000 Newton's to said powder.”
In claim 8 of U.S. Pat. No. 7,175,685, “The silicon pellet for making silicon ingots according to claim 1, said pellet having a diameter of about approximately 14 mm, and a height of about approximately 10 mm.”
In claim 12 of U.S. Pat. No. 7,175,685, “A silicon pellet for making silicon ingots, comprising a dry compacted volume of silicon powder compacted by a force of at least approximately 10,000 Newton's, said powder being free of intentional additives and binders, said pellet having a density of about approximately 60-75% of the theoretical density of elemental silicon, and having a weight of about approximately 2.3 grams, said pellet having a diameter of about approximately 14 mm, and a height of about approximately 10 mm.”
Even the pressure parameter given by U.S. Pat. No. 7,175,685 is 10,000 Newton's, but area of thrust surface of 10,000 Newton's is not mentioned in the full text. It has not pointed out the working pressure of machine, and area of thrust surface of silicon brick as well. The conclusion of the full text of U.S. Pat. No. 7,175,685 is that the technical solution of this patent is to produce cylindrical silicon brick having a diameter of about approximately 14 mm, and a height of about approximately 10 mm by dry pressing method. This is a typical pressing technology similar to drug pressing.
This manner of dry pressing with single-direction, is a simple and vivid simulating technology of medicinal powder pressing. A certain amount of silicon powder is charged into cylindrical die. The top or the bottom of cylindrical die is pressurized to obtain a cylindrical silicon brick having a increased density, which is known as single-direction or bidirectional dry pressing. However, the increase of silicon density is limited using the single-direction or bidirectional dry pressing method because the range of pressurizing from the pressing machine is low and the force and the manner of pressurizing are limited. However, the pressure transmissibility of silicon powder is poor, which forms a great pressure gradient in the formed silicon brick. The middle area of the silicon powder has low pressure because it cannot be tightly-compacted. Consequently, when taking the formed silicon brick out from the die, it is easily cracked, which restricts the application of silicon brick.
For example, a cylindrical die is filled with silicon powder with average diameter of 10 micrometers, and then opened the die to obtain a cylindrical silicon brick by simulating bidirectional dry pressing on medicinal powder. Uneven pressure on silicon powder makes formed silicon brick with uneven density. For example, density on the top of the cylindrical silicon brick is greater than the middle of the cylindrical silicon brick. There is even a big crack and other defects in the middle portion of the cylindrical silicon brick. The cylindrical silicon brick is still easily cracked and has the problem of flowing silicon dust. If it is charged into a mono-crystal furnace or multi-crystal furnace, the silicon dust will easily flow with the protective gases that will seriously influence the regular production.
The medicinal powder dry pressing method to press silicon powder into silicon brick, yields a silicon brick with the solid density of 0.9 g/cm3 to 1.7 g/cm3. Since the internal structure of the silicon brick is uneven, sometimes there is some silicon powder falling off from the surface of the silicon brick and, sometimes a majority of the silicon brick produced from the cylindrical die is cracked and returns to a state of silicon powder. The silicon bricks can be easily broken and cracked during transportation and delivery, which is harmful to its application in a mono-crystal furnace or multi-crystal furnace.
The compression resistance of a silicon brick sample having a density of 0.9 g/cm3 to 1.9 g/cm3 pressed by single-direction or bidirectional dry pressing method underperforms. The silicon brick can even be cracked by a slight touch of the finger.
Claim 12 of U.S. Pat. No. 7,175,685 describes producing silicon brick without using a binders additive.
The applicant of the invention has developed specific binders for making silicon brick. The specific binders are mixed using a certain amount of polyvinyl alcohol and polyvinyl butyral or polyethylene glycol. They are added to silicon powder having an average diameter of 10 micrometers and a weight ratio of 0.1-10%. A large force (10 MPa to 100 MPa) is pressed on the top and bottom end of cylindrical die by bidirectional dry pressing method. However, the compression resistance of the obtained silicon brick still underperforms.
The compression resistance of silicon brick related to the invention is measured by putting a silicon brick having a certain size into the presser, and determining the compressive stress or pressure when the silicon brick is cracked.
In a simple experiment a modified medicinal powder pressing machine is used to show traditional medicinal powder pressing technology. Silicon powder is charged into a cylindrical die having a diameter of 20 mm and a length of 10 mm, and a pressing force of 10 Mpa is applied in a single-direction on the silicon powder. After pressurizing, the cylindrical silicon brick is taken out from cylindrical die, but the silicon brick is cracked and cannot be tested on its density and compression resistance.
In another experiment using the technology of traditional pressing, silicon powder is added into a cylindrical die having a diameter of 100 mm and a length of 300 mm, and a pressing force of 60 Mpa is applied in a single-direction on the silicon powder. After pressurizing, the silicon brick is removed from the cylindrical die but it is cracked in the middle and there is a small amount of silicon powder falling off the surface. The density deviation between a silicon brick having a volume of 1 cm3 from the pressurizing contacting interface and having a density of 1.68 g/cm3 and a silicon brick having a volume of 1 cm3 at a distance of 60 cm from the pressurizing contacting interface and having a density of 1.33 g/cm3, is about 21%. However, the middle position of cylindrical silicon brick at a distance of 150 cm from the pressurizing contacting interface of silicon brick is cracked. The density of the cracked point is taken as 0, and density deviation of those two silicon bricks is 100%. The sample for compression resistance test is cracked in the process of production and cannot be tested.
Even though U.S. Pat. No. 7,175,685 could press silicon powder into forming constrainedly, its compression resistance so poor, that it cannot compression resistance tested.
Even though the dry pressing technology of U.S. Pat. No. 7,175,685 can obtain silicon brick with density greater than the density of loose silicon powder, the filling property of the silicon bricks produced is poor. Their compression resistance is not optimal, and their internal density is uneven.
The dry pressing technology of U.S. Pat. No. 7,175,685 can obtain silicon brick with density greater than the density of loose silicon powder, but its low compression resistance will cause the problem of flowing silicon dust. Also, silicon brick is easily cracked during the process of discharging, transportation and charging into a mono-crystal furnace or multi-crystal furnace from the die. The dry pressing technology of U.S. Pat. No. 7,175,685 does not completely resolve the problems of flowing silicon dust or cracking silicon brick.
There are no technical suggestions given by U.S. Pat. No. 7,175,685 about preventing silicon powder or silicon brick from contacting the air and producing oxide film.
Fifth, the present invention introduces silicon brick produced by sintering method. The existing technology of CN2008100314982 involves sintering metallurgical silicon with purity of 99.0% to 99.9% at temperature of 1050° C. to 1150° C. The existing technology of JP2004284929A involves sintering at temperature of 1200° C. to 1412° C. at atmospheric pressure of 1000 KPa to obtain silicon brick having a relative density of 99.9% which is used in the semiconductor industry.
With the sintering method it is possible to obtain silicon brick of high density at temperature of higher than 1000° C. or even at temperatures close to the melting point of silicon. The production facility requirement and energy consumption of the sintering method is very high. Thus its production cost is extraordinary high. The compression resistance of silicon brick using this method is close to that of silicon brick having a relative density of 100% and a real density of 2.33 g/cm3 produced by vapor deposition method. Or the compression resistance of high density silicon brick produced by the sintering method is close to that of the waste high-purity silicon brick recycled from semiconductor or solar industry and having the casting process.
TABLE ICompression resistance of diversified silicon bricksin current technical fieldEvery varietyof high-puritysilicon rawRecycled waste siliconBulk, flaky and roughSilicon brick produced bymaterialbrickgraininess silicon bricksintering methodSourceHigh-purity waste siliconThe silicon brick isSilicon brick produced bybrick recycled fromproduced from big siliconsintering method. Forsemiconductor, solarbrick by process of slicingexample, at temperature ofindustry, etc.or being cracked, and the1200-1412° C. atbig silicon brick isatmospheric pressure ofproduced from upstream1000 KPa.manufacturer ofphotovoltaic andsemiconductor industry byvapor deposition methodfor instance.CompressionGood compressionGood compressionGood compressionresistanceresistance. For instance,resistance. For instance,resistance. For instance,60-206 Mpa or higher.60-206 Mpa or higher.55 Mpa or higher.Relative100%100%About 99%.density
Sixth, there are some reports in the existing technology of pressing silicon powder into silicon brick using isostatic pressing as well. For example, the Ceramic Journal Vol. 27 No. 2 June. 2006, “Effects of the amount of water and binder on cold isostatic pressing forming ability and performance of green body” describes a method using a uniform mixture of silicon powder (granularity≦0.044 mm) and polyvinyl alcohol solution (PVA, 5 wt %) in a certain proportion. After sieving, prilling, vibration packing and cold isostatic pressing, the green body is pressed. This article describes research to improve on isostatic pressing by adding polyvinyl alcohol and research on the effects of the amount of water and binder on forming ability and performance of silicon powder green body. However, this reference doesn't describe silicon brick produced from silicon powder by isostatic pressing to be utilized in a mono-crystal furnace or multi-crystal furnace as raw material of silicon crystal growth.
US20070014682 also describes producing silicon brick by dry pressing method with binders and using it in the process of silicon melting. However, US20070014682 isn't related to isostatic pressing technology.